Arthropods

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Chapter 18
Arthropods
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Outline

• Intro
• P. Arthropoda
• SubP. Trilobita
• SubP. Chelicerata
• C. Merostomata
• C. Pycnogonida
• C. Arachnida

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Outline

• SubP. Crustacea (Chapter 19)
• C. Remipedia
• C. Cephalocarida
• C. Branchiopoda
• C. Ostracoda
• C. Maxillopoda
• C. Malacostraca
• SubP. Uniramia (Chapter 20)
• C. Diplopoda
• C. Chilopoda
• C. Pauropoda
• C. Symphyla
• C. Insecta

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Intro

• Arthropodization
• soft cuticle stiffened by deposition of protein
  chitin
• exoskeleton provides protection
• site for muscle attachment thus adjacent segments
  act as levers
• flight/rapid mvmt
• joints of thin/flexible exoskeleton provide
  flexibility
• sequence of molts necessary for growth
• hydrostatic skeleton func. lost
• coelom regressed/replaced by open sinuses
• loss of septa
• development of hemocoel
• loss of closed circulatory system
• jointed appendages
• motile cilia lost
• monophyletic
• otherwise all features arose independently

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P. Arthropoda

• Characteristics
• ¾ all known sp.
• abundance wide ecological distribution most
  diverse animal group
• 1,100,00 arthropod sp. recorded
• fossil history dates to late Precambrian (600
  MYA)
• primitive body plan linear series
  of
  segments w/ jointed appendages
• eucoelomate protostomes
• distinct segments (similar to annelids)
• molecular data indicate different
  
  ancestors
• exoskeleton w/ chitin
• well-developed organ systems

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P. Arthropoda

• bilateral symmetry
• segmented body divided into tagmata
• head/trunk
• head/thorax/abdomen
• cephalothorax/abdomen
• jointed appendages
• often highly modified
• cuticular exoskeleton of protein/lipid/
  chitin/CaCO3
• molted
• complex muscular sys.
• attached to exoskeleton
• striated muscles for rapid mvmt
• smooth muscles for visceral organs
• no cilia

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P. Arthropoda

• many somites fused/combined into specialized
  groups (tagmata)
• appendages often highly specialized for division
  of labor
• 0.1 mm (follicle mite) - 4 m (Japanese crab leg
  span)
• all modes of feeding
• mostly herbivorous
• harmful ? beneficial
• occupy diversity of habitats

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P. Arthropoda

• Characteristics
• reduced coelom
• hemocoel filled w/ blood
• complete digestive tract w/ modified mouthparts
• open ciruculatory system w/ contractile
  ?/arteries/hemocoel
• R via body surface/gills/tracheae/book lungs
• excretory glands (coxal/antennal/maxillary
  glands) or Malpighian tubules
• nervous system w/ dorsal brain w/ rind to double
  nuerve chain of ventral ganglia
• dioecious w/ internal
  fertilization
• oviparous/ovoviviparous/
  viviparous/parthenogenesis
• metamorphosis

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Comparison of P. Arthropoda and Annelida

• Differences (characteristics of Arthropoda)
• lack septa
• tagmatization
• reduced coelom, instead hemocoel
• open circulatory system
• exoskeleton
• jointed appendages
• cmpd eyes w/ well-developed
  sense organs
• lack cilia

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Diversity and Abundance of Arthropods

• Diversity due to diversity of structures/physiolog
  ical patterns
• versatile exoskeleton
• secreted by epidermis
• cuticle highly protective but jointed
• provides mobility
• consists of inner thick procuticle outer thin
  epicuticle
• procuticle exocuticle secreted before molt
  endocuticle secreted after molting
• both layers contain chitin w/ protein
• 40 chitin in procuticle in insects
• 80 chitin in procuticle of crustaceans
• procuticle is lightwt/flexible/provides
  protection against dehydration
• impregnated w/ Ca salts thus very hard in
  lobsters/crabs

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Diversity and Abundance of Arthropods

• cuticle laminated and further hardened by
  sclerotization (cross-linking)
• thin cuticle btwn segments allows mvmt at joints
• cuticle folds inward to line foregut/hindgut/trach
  ea
• does not permit growth
• molting (ecdysis) process of shedding outer
  covering/growing new, larger exoskeleton
• typically molt 4-7x
• wt of exoskeleton limits ultimate body size

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Diversity and Abundance of Arthropods

• segmentation and appendages confer more efficient
  locomotion
• usually each somite bears 1 pair of jointed
  appendages
• segments/appendages modified for various adaptive
  func.
• limb segments hollow levers w/ internal striated
  muscles
• appendages may func. in sensing/food
  handling/walking/swimming
• air piped directly to cells
• terrestrial arthropods use tracheal system where
  O2 delivered directly to cells via air tubes
• aquatic arthropods respire via various forms of
  gills
• highly-developed sensory organs
• simple light sensitive ocelli ? compound mosaic
  eye
• touch/smell/hearing/balancing/chemical reception

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Diversity and Abundance of Arthropods

• complex behavior patterns
• complex and organized activities
• most behavior innate/unlearned
• some learned
• ? competition via metamorphosis
• metamorphic changes result in different
  larval/adult stages
• larvae/adults eat different foods/occupy
  different habitat

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SubP. Trilobita

• Trilobites
• Characteristics
• arose prior Cambrian, flourished, then extinct
  200 MYA
• trilobed body due to pair of longitudinal grooves
• bottom dwellers, probably scavengers
• 2-67 cm
• roll up
• chitin exoskeleton w/ CaCO3
• body divided into 3 tagmata
• cephalon/trunk/pygidium
• cephalon w/ antennae/cmpd eyes/mouth/
  jointed appendages
• trunk varied somites
• pygidium fused into plate
• pair biramous appendages on each
  somite except last
• 1 branches fringed (gill?)

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SubP. Chelicerata

• Giant water scorpions/horseshoe
  crabs/mites/tickes/spiders
• Characteristics
• 6 pairs of cephalothoracic appendages
• 1 pair chelicerae (mouthparts)
• 1 pair pedipalps
• 4 pair of legs
• lack antennae/mandibles
• most suck liquid food from prey

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SubP. Chelicerata C. Merostomata

• SubC. Euryptida (giant water scorpions)
• largest of all fossil arthropods (3 m)
• fossils in rocks from Ordovician to Permian
• resemble marine horseshoe crabs/terrestrial
  scorpions
• 6 pairs of appendages
• abdomen w/ 12 segments spike-like telson
• head w/ 6 fused segments
• simple and compound eyes
• chelicerae
• pedipalps
• 4 pairs of walking legs

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SubP. Chelicerata C. Merostomata

• SubC. Xiphosurida (horseshoe crabs)
• modern horseshoe crab nearly
  unchanged from ancestors
  in Triassic
• 5 living sp. in 3 genera
• mostly shallow water
• body structure
• unsegmented shield (carapace) covers body in
  front of broad abdomen/telson
• cephalothorax w/ 5 pairs walking legs 1 pair
  chelicerae
• abdomen w/ 6 pairs of broad/thin appendages fused
  in median line
• book gills exposed on some abdominal appendages
• carapace w/ 2 compound 2 simple eyes

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SubP. Chelicerata C. Merostomata

• walk w/ walking legs, swim w/ abdominal plates
• feed at night on worms/small molluscs
• during mating season, come to shore at very high
  tide to mate
• ? burrow into sand to lay eggs
• ? add sperm before ? covers eggs
• young larvae hatch/return to sea at next very
  high tide
• segmented larvae resemble trilobites

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SubP. Chelicerata C. Pycnogonida

• Sea spiders
• Characteristics
• few mm to larger sizes
• all have small/thin bodies
• 1,000 sp.
• some sp. duplicate somites
• may have five or six pairs of legs
• some ? may have subsidiary pair of legs
  (ovigers) to carry
  developing eggs
• many also have chelicerae and palps
• mouth, at the tip of a proboscis, sucks juices
  from
  cnidarians/soft-bodied animals